Goal: To change the length of a wire doublet via remote in-line switches (i.e. without traps).

Why?

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Potential constraints - rf arc over

Probably the biggest issue... can be handled at low power fairly easily. At high power much harder.

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- contact current ratings

Probably not a huge deal for any but the shortest antennas loaded very efficiently.

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- weight- wind resistance

Similar issues to traps, maybe a bit better if you can keep the voltages down, like in low power situations.

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There have been a couple of cool designs in QST that do this... one with logic circuits and relays, the other with pneumatic switches. But I think they're 'cool' mainly from a creativity standpoint. Traps are a very good solution for a lot of reasons. I've done antennas with relay switching; I have one I'm using right now:

and it has occasional problems on RX. It's relatively manageable, especially after I sprayed relays with contact cleaner but it's not super reliable. And my relays don't have to stand off the kind of voltage you see at the end of a resonant section.

I've built a lot of antennas with switches in them, even going pretty fancy (http://n3ox.net/projects/stepperswitch/) and over time I've come to prefer antennas that don't require any switching. Just eliminates a lot of points of failure. The new rotary vertical switch is very reliable so far but it has some power handling problems on 160m. Arcs from the switch contact to the switch shaft above about 450W. If I need a single-vertical low band solution at another QTH I am probably going to try to build a super trap vertical with very high quality traps.

Any time you're relying on switch contacts at a high voltage point you may be in for a tough time. And you have to keep a controller circuit alive. No problem if it's a manual unit that's just a bunch of mechanical switches; more of a problem if you want to computer control it.

How about feeding a fixed wire doublet with parallel feedline and switching the length of the feedline to achieve system resonance? Switching is a lot easier on the ground close to the shack than 40 ft. in the air. The voltages encountered at the ends of a resonant dipole are very high, e.g. over 1000 volts for 100 watts, which exceeds the voltage rating of most affordable relays.

For instance, I'm about to put up a HEDZ (Half Extended Double Zepp) primarily for 80m/40m operation. EZNEC sez a 155 ft. dipole fed with 88 ft. of 450 ohm ladder-line will be resonant on both 80m and 40m. As a bonus, it is also near resonance on 17m and 15m. If one chooses to vary the length of the ladder-line between 73 ft. and 89 ft., all-band-HF coverage is possible without a tuner. Here are the feedline lengths predicted by EZNEC for a 155' dipole.

My motivation was to be able to switch my 94 footer into a 44 footer and emphasize broadside radiation on 20 thru 10 meters.

OK... that makes sense. How much power you running?

I would consider something like pneumatics in this case because choking electrical control lines at the feedpoint of a multiband doublet could be very difficult. It's different at the center of a switched resonant antenna because there you have a low impedance low voltage feedpoint.

If you go with something like a latching vacuum relay (Max Gain Systems has some), you could potentially use a second high voltage relay (maybe a more conventional one) to disconnect the control lines at the feedpoint.

The voltages across a 0.5pF set of switch contacts in your application is nearly 2kV at 1500W with a 14AWG 94 foot wire 30 feet over ground, so you'll want to find something with at LEAST a 2kV RF working voltage if you run legal limit. It's roughly 500V at 100W. If you use regular relays and not vacuum relays you should significantly oversize and may have to experiment because the voltage ratings probably won't be at RF.

You need to keep contact capacitance very, very low. A few pF on 10m is enough to get substantial current in the "switched off" end sections and make the secondary lobes dominant. If you use relays, the coil to contact capacitance should be considered carefully too.

I think it's going to be a little hard to get an electrically foolproof system that actually does what you want it to here. If you could use something like pneumatically or mechanically actuated knife switches that open up an inch gap or so, that might be the ideal in terms of electrical performance.

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